If we assume that the interaction potential of a dust particle in a plasma is of a Yukawa type, then we can determine the particle charge and the screening length from its trajectory during central collisions. We present a theoretical approach taking into account, that the particles are horizontally confined and that friction may influence the trajectories. In addition we show former results (1) based on this theory in the limit of negligible friction and confinement forces. ©1998 American Institute of Physics.